Dispersion strengthening of aluminum alloys by reaction of unstable oxide dispersions

ABSTRACT

A method for preparing dispersion-strengthened metal consisting predominantly of aluminum metal as the metal matrix is provided comprising ball milling said metal as a powder with an unstable oxygen compound whose free energy of formation per gram-atom of oxygen at 500* C. is less than that of the oxide of said metal matrix and which undergoes reaction with said metal matrix at said temperature to form a stable oxide dispersant in a hermetically sealed mill under an inert atmosphere in the presence of a grinding liquid. The resulting flake metal having homogeneous dispersions of the included oxide is then reacted in a nonoxidizing environment at a temperature of about 500* C., pressed, and hot extruded to form the dispersion strengthened product. Extruded bar material containing only 4 weight percent oxide dispersant exhibited super elevated temperature strength when compared to conventional sintered aluminum product containing up to 15 weight percent oxide dispersant. Another alloy showed improved low strain rate fracture ductility with good strength at high temperature when compared to sintered aluminum products.

Unite States Patent [72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee Joseph P. Hammond Knoxville, Tenn.

Oct. 18, 1968 ept- 1 1 1 The United States of America as represented by the United States Atomic Energy Commission [54] DISPERSION STRENGTHENING OF ALUMINUM ALLOYS BY REACTION OF UNSTABLE OXIDE DISPERSIONS 2 Clalmn, 4 ltruwlng Flgn.

[52] U.S.Cl

[51] lnt.Cl [50] Field ofSearch .1

References Cited UNITED STATES PATENTS S Iayter et a].

2,973,570 3/1961 Nachtman 75/206 OTHER REFERENCES Goetzel, Treatise on Powder Metallurgy 1949 pg. 4 Kubaschewski, et al., Metallurgical Thermochemistry Pergamon Press, 1967, pp. 421, 427

Primary Examiner-Benjamin R. Padgett Assistant Examiner-B. H. Hunt Attorney-Roland A. Anderson ABSTRACT: A method for preparing dispersion-strengthened metal consisting predominantly of aluminum metal as the metal matrix is provided comprising ball milling said metal as a powder with an unstable oxygen compound whose free cncrgy of formation per gram-atom of oxygen at 500 C. is less than that of the oxide of said metal matrix and which undergoes reaction with said metal matrix at said temperature to form a stable oxide dispersant in a hermetically sealed mill under an inert atmosphere in the presence ofa grinding liquid. The resulting flake metal having homogeneous dispersions of the included oxide is then reacted in a nonoxidizing environment at a temperature of about 500 C., pressed, and hot extruded to form the dispersion strengthened product. Extruded bar material containing only 4 weight percent oxide dispersant exhibited super elevated temperature strength when compared to conventional sintered aluminum product containing up to 15 weight percent oxide dispersant. Another alloy showed improved low strain rate fracture ductility with good strength at high temperature when compared to sintered aluminum products.

' PATENTHJ SEP2I I9?! 3,607,254

sum 1 0F 2 INVENTORI. Joseph 1? Hammond ATTORNEY.

PATENTEU SEP21 I97! 3,607,254

SHEET 2 OF 2 INVENTOR.

Joseph 1? Hammond BY ATTORNEY DISPERSION STRENGTHENING OF ALUMINUM ALLOYS lBY REACTION OF UNSTABLE OXIDE DISPERSIONS BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the U.S. Atomic Energy Commission. It relates generally to dispersion strengthened metals and more particularly to dispersion strengthening of aluminum alloys by reaction of unstable oxide dispersions.

It is well known in the art that aluminum matrices having a controlled quantity and fineness of oxide particles dispersed therein have superior strength properties at elevated temperatures. These materials, commonly termed SAP (sintered aluminum products), require from to weight percent oxide dispersed in the matrix thereof in order to achieve the desired strength characteristics. For optimum dispersion hardened alloy certain structural conditions are sought, namely: (1) the dispersant particles should be small, 001 to 0.05 micron, and they should be uniformly and closely spaced, 0.1 to 0.5 micron, (2) the dispersed phase should be hard, (3) the interfacial energy at particle boundaries should be low, (4) the free energy of formation of the dispersed phase should be high, and (5) the solubility and diffusivity of the dispersed phase components in the matrix should be low, if not negligible.

The first two of these criteria are probably most connected with high-temperature strength as they relate to the obstruction of dislocation flow. The latter three items are indirectly related to high-temperature strength in that they are vital to microstructural stability at elevated temperature and thus the assurance that condition one can be maintained to higher and higher temperature. To sum up, the size of the hard particles and, more especially, their interparticle spacing are features of major concern in developing improved dispersion strengthening aluminum matrix alloys.

A principal shortcoming of the conventional SAP process is that the open milling method used to introduce the oxide by allowing the aluminum to react with the air as it is ground, gives insufficiently fine and uniform dispersions. In order to achieve the optimum interparticle spacing, probably the most important strength-related variable, extremely long ball milling times are required. Unfortunately, however, the amount of oxide formed increases will milling time, and materials with desired strength generally have too high an oxide loading for optimum toughness and ductility.

Extensive development has been underway in recent years in this country and abroad to improve the quality of SAP for heavy-water-moderated, organic-cooled reactors. However, SAP materials have been generally considered deficient in that their high-temperature mechanical properties were not sufficiently high and uniform and low strain rate fracture ductility was inadequate. It was widely regarded that these properties could be improved by better controlling the size and distribution of the oxide dispersant. In the interest of neutron economy, any alloying had best be confined to elements of low thermal neutron cross section, e.g., beryllium, magnesium, aluminum, silicon, yttrium, cerium, oxygen, and carbon.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method for achieving extraordinarily fine dispersions of an oxide in a metal matrix consisting predominantly of essentially pure aluminum metal which is independent of the amount of oxide introduced. Applicant has discovered that a very strong product is formed by high-energy ball milling a metal powder mixture of aluminum or an aluminum alloy with a preselected amount of an ultrafine unstable oxygen compound in a hermetically sealed mill under an inert atmosphere in the presence of a grinding liquid and thereafter reacting at a temperature of about 500 C. the resulting flake metal in nonoxidizing environment to form a stable oxide dispersant in the metal and finally extruding the reacted mass into a sintered final product. By ultrafine, unstable oxygen compound it is in- Aluminum-5.5 weight percent magnesium alloy extruded bar" tended to refer herein to an unstable oxygen-bearing compound whose free energy of formation per gram-atom of oxygen at 500 C. is less than that of the oxide of the aluminum metal matrix, A1 0 (-1l5 Kcal per gram-atom of oxygen), and which undergoes reaction with the metal matrix at 500 C. to form a stable oxide dispersant. Suitable examples of such compounds include, but are not limited thereto, mo, (5 l Kcal per gram-atom of oxygen), Cr 0 (-75 Kcal), SnO, (5l.5 Kcal), amorphous SiO Kcal), or colloidal boehmite (AlOOH). An aluminum-l 1 weight percent cerium alloy with 4 weight percent ultrafine SiO ground in it for 24 hours and subsequently reacted and worked in accordance with this invention had a low strain rate yield strength at 450 C. of 14,720 p.s.i. compared with 10,720 psi. for a conventional extruded SAP material of 11 weight percent A1 0 stock strengthened with 4 weight percent colloidal AIOOH as additive exhibited a low strain rate yield strength at 450 C. comparable to that of extruded SAP material of 6 weight percent oxide (8,800 psi. as against 8,650 psi. yet it displayed fracture ductilities (percent total elongation) about twice that of the SAP material. Like materials processed similarly but ball milled for 48 hours gave even more outstanding mechanical properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a high-magnification electron transmission photograph of aluminum-55 weight percent magnesium alloy hardened with 4 weight percent AlOOH additive.

FIG. 2 is a high-magnification electron transmission photograph of pure aluminum strengthened with 4 weight percent ultrafine amorphous SiO additive.

FIG. 3 is a high-magnification electron transmission photograph of the amorphous SiO additive used for forming stable oxide dispersions in aluminum alloys.

FIG. 4 is a high-magnification electron transmission photograph of aluminum-l1 weight percent cerium powder in which 4 weight percent amorphous Si0 has been imparted by ball milling for 24 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENT It will be appreciated that while the present invention is hereinafter described with particular reference to unstable amorphous SiO particles and boehmite (AlOOH) fibers as the unstable oxygen compound the invention is broadly applicable to any unstable oxygen compound as previously defined. In a first step of the process aluminum metal or aluminum alloy material is ground with the unstable oxygen compound in a ball mill. The metal starting material should be in particulate form, preferably in one of the forms known as atomized, shotted, or splat-cooled powder. Aluminum alloy powders splat-cooled in an argon atmosphere were used in the present work. It is distinctive to this invention that the oxide dispersant be added as an unstable oxide (i.e., one whose free energy of formation per gram-atom oxygen at 500 C. is less than that of the oxide of the metal matrix and which undergoes reaction with the metal matrix at 500 C. to form a stable oxide dispersant) and that the milling be carried out in a hermetically sealed drum under protective materials so as to eliminate oxide buildup during grinding and thus control the content of oxide dispersant in the final product. These requirements will be recognized as being completely different from the prior art in SAP processing. While crystalline SiO (quartz) is normally quite stable at room temperature, applicant has found that SiO in the form of an ultrafine amorphous or glass SiO is unstable and undergoes reaction at around 500 C. with the metal matrix material to form the desired oxide dispersant in situ. In an alternative embodiment applicant employs ultrafine fibers of boehmite (AlOOH), measuring approximately 50 angstroms in diameter and about 1,000 angstroms long. This fibrillar boehmite is unstable and transforms to gamma alumina without loss of physical structure on heating in air at around 450 C. When ground into aluminum alloy powder it is also unstable, and in an aluminum-5.5 percent magnesium powder was shown to transform to stable globular oxide particles of unidentifiable crystal structure.

It should thus be apparent that the gist of applicants discovery is that aluminum alloy-oxide dispersions can be prepared with desired control of quantity, size, and distribution of the dispersant by providing for it by the addition of an unstable oxygen compound at the formulation stage of the process. The requisite fine and uniform final dispersion is insured without attendant increase in oxide content as encountered with the prior art processes, by grinding the unstable oxide additive into the metal matrix powder in a hermetically sealed ball mill backfilled with protective argon gas. An organic milling liquid containing a surfactant is used to aid the grinding. The surfactant helps to prevent powder seizing during grinding and also serves as a binder in subsequent drying and powder-working operations to prevent any tendency for the oxygen-carrying dispersant to segregate before reacting in in place.

While the quantity of unstable oxygen compound used may be varied over a considerable range, applicant has found that around 4 weight percent additive gives best strength with desirable toughness and ductility. Neglecting oxygen contamination and assuming that the intended oxide dispersant were A1 with a density of 3.5 gr./cc., 3.54 volume percent oxide in the fabricated alloy theoretically would be achieved by adding 4 weight percent of either SD; or A100H. Although it is true that magnesium, cerium, beryllium or yttrium of a matrix powder probably concentrates in the formed oxide and the starting matrix powders contain about one-fourth weight percent oxygen to begin with, the above gives an approximate relationship between the amount of additive used and the oxide formed.

After dispensing appropriate quantities of matrix and additive powders in the ball mill the grinding liquid is added. Of a number of solvents and surfactants investigated for making the grinding liquid, one-fourth to one-half weight percent stearic acid (based on weight of the matrix and metal oxide powders combined) dissolved in petroleum ether proved to be the most desirable liquid. The amount of liquid selected was just sufficient to give maximum splashing during ball milling. Other grinding materials examined included hexane, 1,1,2- trichloro-l ,2,2-trifluoroethane, and kerosene as solvents, and camphor, methyl silicon, and aluminum and magnesium stearates as surfactants. Attractive features of the grinding liquid selected included easy removal of the petroleum ether from the ground powders by simple evaporation and minimal contamination of the charge powders by exposure to the liquid. The main contaminant from this liquid was carbon which was picked up in an amount between one-half and one percent. This amount of carbon contamination did not appear to be objectionable and may have contributed to mechanical properties.

The ball mill for milling the powder mixture may comprise a conventional rotary drum or a planetary high-impact-type mill provided they can be hermetically sealed by gasketing for maintenance of the protective atmosphere. A steel drum hard surfaced on its interior with a Hadsfield-type metal and containing two heavy overlay weld passes 90 apart for agitating the balls, which are tungsten carbide, is quite effective.

Milling times may vary widely and will depend upon the degree of fineness of dispersion desired and the grinding efficiency of the mill used. It is apparent that the longer the milling times the finer the dispersion of the oxide will be in the metal matrix. A milling time of 24 hours is quite satisfactory for preparing suitable aluminum alloy flake containing 4 weight percent of either ultrafine SiO- or A 1 OOH fibers.

After milling, the solvent of the grinding liquid is removed from the milling drum by evaporation. The powders are then dry milled for about 15 minutes longer, which breaks up the cake and makes the powders easily extractable from the mill.

At this stage the flake aluminum or aluminum alloy material contains homogeneous dispersions of the included oxide having an extremely close interparticle spacing. Referring to FIG. 4 the fineness and uniformity of the dispersion even at this stage are apparent. Analysis of powders ground by this process using the diffusion scattering X-ray technique gave crystallite sizes for the aluminum phase of around 300 angstroms as compared to 1,300 to 1,600 angstroms for commercial SAP flake material.

The present powder milling process has an important economic advantage over that used in the trade that warrants mentioning here. The major share of the cost of powder preparation in the conventional SAP process stems from the need to remove kerosene and copious amounts of stearic acid used as surfactants (around 3 weight percent) from the powders after completing the milling. This grinding liquid has relatively low inflammability which constitutes a safeguard against explosion during grinding. However, it must be subsequently removed to hold carbon contamination to within a tolerable level. This is done by filtration followed by washing of the powders with a volatile and usually more inflammable96 solvent (acetone is frequently used). in the present method powder washing is eliminated. Since powder handling is done within an argon-filled dry box there is no explosion hazard.

The powders are then reacted by heating to an elevated temperature. Temperatures of about 500 C. are suitable and it is preferable to conduct this step under vacuum (10- mm. hg.) although some of the powders were satisfactorily prepared by reacting them under argon.

Following the vacuum heat treatment the flake material is cold pressed into compacts at a pressure of 33 p.s.i. or higher. This cold compaction provides densities of .about 96 percent of theoretical. The surface structure appears to be closed, allowing the compacts to be extruded bare in air, if desired, without undesirable effects. It should be apparent that other powder metallurgical techniques may be employed in preparing these compacts. For example, nonreacted powders may be cold pressed followed by reaction sintering in vacuum or cold pressed and then reacted by vacuum hot pressing.

As a final step the pressed compacts are shaped into desired bodies, such as extruded or swaged bar or rolled sheets. Where aluminum or aluminum alloy bars are desired compacts may be hot extruded at about 500 C. employing a conventional die press at pressures of 160,000 to 200,000 p.s.i.

Having described the invention in general fashion, the following examples are given to indicate with greater particularity the process parameters and techniques.

EXAMPLE 1 An aluminum5.5 weight percent magnesium alloy (100 grams), designated as Alloy No. 3, in the form of- 100 mesh powder was placed in a ball mill along with 4 weight percent fibrillar boehmite (A1001-1) obtained commercially from the E. I. du Pont de Nemours & Company. This boehmite material was about 50 angstroms in diameter and 1,000 angstroms long. The ball mill comprised a gasketed, stainless steel drum hard surfaced with Hadsfield metal and containing two weld overlays apart for agitating the charge. The drum, which was 7 inches in inside diameter, contained about one-tenth of its volume in nine-sixteenth-inch diameter tungsten carbide balls and was rotated at 88 rpm. About ml. of petroleum ether with 0.5 weight percent dissolved stearic acid (based on weight of powder mixture) was added to the charge as a grinding liquid. The mixture was milled for a period of 24 hours in the drum, which was hermetically sealed and provided with a protective argon atmosphere. After milling the grinding liquid was evaporated out under ambient conditions in an argonfilled dry box, requiring 2 to 3 hours. Then the flake material was reacted by heating to 500 C. in a vacuum of 10' mm. of hg. Finally, the reacted flake material was cold compacted at 33 tsi. in a steel die and hot extruded at 500 C. to a 20:1 reduction ratio at 160,000 p.s.i. pressure into a 54-inch diameter rod.

For comparison purposes commercial SAP rod materials containing 6 and l 1 weight percent A1 0 designated as Alloy No. l and Alloy No. 2, respectively, were tested. These materials are known commercially as XAP-001 and SAP 895, respectively. The powders were ground in an open ball mill using a proprietary grinding liquid. They were then consolidated and extruded at a 25:1 reduction ratio.

The rods were tested on an Instron tensile tester at room temperature at a strain rate of 0.02 (in./in. min.) and at 450 C. at strain rates of 0.02, 0.002, and 0.0002 (in./in. min"), the results are shown in the accompanying table. It will be observed that at 450 C. at the lowest strain rate the present alloy (No. 3) had a yield strength of 8,800 p.s.i. which was equivalent to the conventional SAP containing 6 weight percent A1 0,, No. 1, (8,650 p.s.i.). Yet the present alloy exhibited a total elongation of about twice the SAP materials.

An electron transmission photograph of this alloy is shown in FIG. l and illustrates the ultrafine and uniform dispersion of oxide particles. The average size of the oxide particles is estimated to be about 0.02 micron. The fact that the particles are globular whereas the oxygen compound introduced was fibrous supports the principle that the dispersant is formed in situ by an oxidation reduction reaction.

EXAMPLE II In another run, designated as Alloy No. 4, 100 grams of aluminum--l 1 percent (2.3 atomic percent) cerium splat was milled in the hermetically sealed drum described in example I under argon along with 4 weight percent amorphous SiO The SiO particles were perfectly round and had a reported average diameter of 140 angstroms (see FIG. 3). This amorphous or glassy SiO material was obtained commercially from Vitro Laboratories of West Orange, New Jersey.

After ball milling for 24 hours in the presence of 100 ml. of the petroleum ether-0.5 percent stearic acid grinding liquid the mixture (the flake of which is illustrated in FIG. 4) was dried at 27 C. for 3 hours and heated in argon at 450 C. for 2 hours. The resulting flake material was cold pressed at 33 t.s.i. into lA-inch compacts and hot extruded at 500 C. to a :1 reduction ratio under a pressure of around 200,000 p.s.i.

The rod was tested as in example I and the results are given in the accompanying table. It will be seen that this alloy, although containing only 4 percent oxide, exhibited elevated temperature strengths substantially in excess of those for SAP of l 1 percent oxide (Alloy No. 2). Analysis of this alloy by X- ray diffraction failed to show the SiO: additive, but revealed an unidentifiable second phase, indicating that the additive had reacted.

EXAMPLE III In another run, designed Alloy No. 5, 100 grams of high-purity aluminum splat was milled in the hermetically sealed drum described in example I under argon along with 4 weight percent amorphous Si0 particles of about 140 angstroms average diameter size. After ball milling for 24 hours in the presence of ml. of the petroleum ether-0.5 percent stearic acid grinding liquid, the mixture was dried for 3 hours and heated in argon at 450 C. for 2 hours. The resulting flake material was cold pressed at 33 t.s.i. into lA-inch compacts and hot extruded at 500 C. to a 20:1 reduction ratio under a pressure ofaround 120,000 p.s.i.

The rod was tested as in example I and the results are given in the accompanying table, Alloy No. 5. Although the percent elongations for this alloy are not good, it gives strengths comparing favorably with SAP of l 1 weight percent oxide (Alloy No. 2).

An electron transmission photograph of this alloy is shown in FIG. 2 and illustrates the exceedingly fine and uniform dispersion in this alloy. The average size of the oxide particles is estimated to beabout 0.03 micron.

EXAMPLE IV In another run, designed Alloy No. 6, 100 grams of aluminum5.5 percent magnesium powder (l00 mesh) was ground with 4 weight percent ultrafine SiO A) in the same fashion as for Alloys 3, 4, and 5 except that the milling time was twice as long, 48 hours. Again the powders were dried in argon and reacted in vacuum at 500 C. The powder mixture was pressed to l%-inch-diameter pellets and hot extruded at a 20:1 reduction ratio at 550 C. under about 200,000 p.s.i. pressure.

Tensile specimens prepared from rods of this material were tested at 450 C. at a strain rate of 0.02 in./in. min and the results are given in the accompanying table. It will be observed that strength values even higher than for the three previous experimental alloys were achieved, owing presumably in part to the longer grinding time.

EXAMPLE V In another experiment, designated Alloy No. 7, 100 grams of aluminum-l.75 percent (5 atomic percent) beryllium splat was ground with 4 weight percent ultrafine SiO again for the longer period of 48 hours. The powder-treating and metalextruding steps were identical to those of Alloy No. 6. Equally high-strength values and good ductility were obtained for this alloy.

It is noteworthy that the alloying elements of the foregoing alloys are of low cost with the exception of cerium, and they have very low thermal neutron absorption cross sections. The aluminum-5.5 percent magnesium matrix powder of Alloy No. 6 is a solid solution alloy, whereas the aluminum-l l percent (2.3 atomic percent) cerium and the aluminum-1.75 percent (5 atomic percent) cerium and the aluminum-l.75 percent (5 atomic percent) beryllium materials are eutectic alloys. The oxide formers of the latter two alloys, cerium and beryllium, exist as intermetallic compounds of aluminum and, since the powders were prepared by splat cooling, they are dispersed very finely as a eutectic mixture. Thus the alloying elements are homogeneously distributed for reaction with the imbedded Si0 particles. Fortunately, the eutectics in these alloys occur at compositions which afford appropriate amounts of alloying element for reacting with oxygen made available by the unstable additive.

TABLE.COMPA RISON OF EXPERIMENTAL ALUMINUM ALLOYS WITH CONVENTIONAL SAP MATERIALS Milling test Yield Ultimate Strain strength tensile Total Alloy Time Temp. rate 0.2% oflset strength elonga- No. Description (hrs.) C.) (minr (p.s.i.) (p.s.i.) tion 1 XAP (6% A120 24-29 0. O2 32, 000 38, 270 I1. 5 450 0. 02 I0, 280 II, 280 2. 7

2 SAP (11% A1103) 24-2!) 0. 02 33, 310 40, 220 9. l 450 (l. 02 13, 490 14, 650 2. 0

3 Expvrliiimiliil Al 5.59;, Mgpuwdurwltli 4,,',Alt)()ll it '.'-i ."J 0. U2 5.5,13-10 (II, 530 ii. 1'. 24 451] 0. 02 II, 440 II, 770 5. 45

Table Continued Milling test Yield Ultimate Strain strength tensile Total Alloy Time Temp. rate 0.2% ofiset strength elonga- No. Description (hi-s.) C.) (minr (p.s.i.) (p.s.i.) tion 4 Experimental Al11% Ce splat with 4% S102 t 24 24-29 0.02 60, 820 76, 300 6. 6 24 450 0. 02 18, 838 19, 411 0.59

5 Experimental pure Al splatwith 4% SiO; 24 450 0.02 13, 887 13, 970 0. 9 24 450 0.002 11,460 11, 625 0.26

6 Experimental Al5.5% Mg powder With 4% SiO2 48 450 0. 02 20, 736 20, 903 2. 51

7 Experimental Al-1.75% Be splat with 4% SiOg 48 450 0.02 19, 262 20, 351 2. 9

I claim:

average diameter and (ii) fibrillar AlOOl-l of substantially 50 angstroms diameter and L000 angstroms length, and (c) a grinding liquid; (2) heating the resulting mixed powder in either (a) an inert atmosphere or (b) a vacuum to about 500 C. to convert, in situ, said unstable compound into a stable oxide; and 3) compacting said powder to a desired shape.

2. The method of claim 1 wherein said unstable oxygen compound comprises about 4 weight percent. 

2. The method of claim 1 wherein said unstable oxygen compound comprises about 4 weight percent. 